Multi-layer structure to increase crystalline temperature of a selector device

ABSTRACT

In some embodiments, a semiconductor device is provided. The semiconductor device includes a first amorphous switching structure disposed over a first electrode. A buffer structure is disposed over the first amorphous switching structure. A second amorphous switching structure is disposed over the buffer structure. A second electrode is disposed over the second amorphous switching structure, where the first and second amorphous switching structures are configured to switch between low resistance states and high resistance states depending on whether a voltage from the first electrode to the second electrode exceeds a threshold voltage.

BACKGROUND

Many modern day electronic devices contain electronic memory. Electronic memory may be volatile memory or non-volatile memory. Non-volatile memory is able to store data in the absence of power, whereas volatile memory is not. Some examples of next generation electronic memory include resistive random-access memory (RRAM), phase-change random-access memory (PCRAM), magnetoresistive random-access memory (MRAM), and conductive-bridging random-access memory (CBRAM). In some next generation electronic memory, selector devices are coupled to memory cells to mitigate the negative effects (e.g., sneak paths) associated with scaling down the feature size of next generation electronic memory.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1A-1B illustrate various views of some embodiments of a memory device having a plurality of semiconductor devices each having a buffer structure disposed between a first amorphous switching structure and a second amorphous switching structure.

FIGS. 2A-2B illustrate various views of some alternative embodiments of the memory device of FIG. 1 in which the threshold selector has additional buffer structures and an additional amorphous switching structure.

FIG. 3 illustrates a cross-sectional view of some embodiments of an integrated chip (IC) comprising a memory device having a plurality of semiconductor devices each having a plurality of buffer structures and a plurality of amorphous switching structures.

FIGS. 4-14 illustrate a series of cross-sectional views of some embodiments for forming an integrated chip (IC) comprising a memory device having a plurality of semiconductor devices each having a plurality of buffer structures and a plurality of amorphous switching structures.

FIG. 15 illustrates a flowchart of some embodiments of a method for forming an integrated chip (IC) comprising a memory device having a plurality of semiconductor devices each having a plurality of buffer structures and a plurality of amorphous switching structures.

DETAILED DESCRIPTION

The present disclosure will now be described with reference to the drawings wherein like reference numerals are used to refer to like elements throughout, and wherein the illustrated structures are not necessarily drawn to scale. It will be appreciated that this detailed description and the corresponding figures do not limit the scope of the present disclosure in any way, and that the detailed description and figures merely provide a few examples to illustrate some ways in which the inventive concepts can manifest themselves.

The present disclosure provides many different embodiments, or examples, for implementing different features of this disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

Some next generation memory comprises a plurality of resistive switching cells. The resistive switching cells define a memory array and are arranged in a plurality of rows and a plurality of columns (e.g., in a cross-bar array). A plurality of first conductive lines are disposed beneath the memory array, and a plurality of second conductive lines are disposed over the memory array. The first conductive lines each extend along an individual column in the memory array and are each coupled to the resistive switching cells in the individual column. The second conductive lines each extend along an individual row in the memory array and are each coupled to the resistive switching elements in the individual row. To select an individual resistive switching cell (e.g., to read/write/erase the cell), a first bias is applied to a first conductive line coupled to the individual resistive switching cell, while a second bias is applied to a second conductive line coupled to the individual resistive switching cell. To improve the sensing margin (e.g., the difference in current between read state “1” and read state “0”), partial biasing schemes (e.g., ½ bias and ⅓ bias schemes) are utilized by applying a partial bias to unselected first conductive lines and unselected second conductive lines.

One challenge with next generation memory is the unintentional activation of unselected resistive switching cells. Due to sneak current and/or the partial biasing of unselected cells, an unselected cell may be exposed to a bias that is sufficient to activate the unselected cell. Accordingly, to prevent the unintentional selection of unselected devices, select devices may be coupled to resistive switching cells (e.g., to obtain a more nonlinear I-V characteristic of the resistive switching cell). In various instances, the select devices comprise an ovonic threshold switching (OTS) layer. The OTS layer is formed in an amorphous state and is configured to switch between a high resistance state and a low resistance state depending on whether a bias applied across the select device exceeds a threshold bias.

One challenge with utilizing an OTS layer in a select device is integrating the formation of the OTS layer in typical back-end-of-line (BEOL) fabrication. During BEOL fabrication, an interconnect structure comprising the select devices may be formed over a semiconductor substrate. To form the interconnect structure, the semiconductor substrate is exposed to a relatively high BEOL fabrication temperature (e.g., about 300° C. or about 400° C.). However, the BEOL fabrication temperature may be greater than the crystallization temperature of the OTS layer (e.g., the temperature at which the OTS layer crystallizes). If the OTS layer crystallizes, the OTS layer may be formed having a low resistance state and the select device may not prevent unintentional activation of unselected cells.

Various embodiments of the present disclosure are directed toward a semiconductor device having a buffer structure disposed between a first amorphous switching structure and a second amorphous switching structure to increase the crystallization temperature of the first and second amorphous switching structures. In various embodiments, a first amorphous switching structure is arranged over a first electrode. A buffer structure is arranged over the first amorphous switching structure. A second amorphous switching structure is arranged over the buffer structure. A second electrode is arranged over the second amorphous switching structure. The first and second amorphous switching structures are configured to switch between low resistance states and high resistance states depending on whether a voltage from the first electrode to the second electrode exceeds a threshold voltage.

The buffer structure reduces the individual thicknesses of the first amorphous switching structure and the second amorphous switching structure. By reducing the individual thicknesses, respective crystallization temperatures of the first amorphous switching structure and the second amorphous switching structure are increased. Thus, the first amorphous switching structure and the second amorphous switching structure may have a crystallization temperature that is greater than the BEOL fabrication temperature. Accordingly, the first amorphous switching structure and the second amorphous switching structure may be integrated into an interconnect structure formed during BEOL fabrication.

FIGS. 1A-1B illustrate various views of some embodiments of a memory device 100 having a plurality of semiconductor devices each having a buffer structure disposed between a first amorphous switching structure and a second amorphous switching structure. FIG. 1A illustrates a perspective view of some embodiments of the memory device 100. FIG. 1B illustrates a cross-sectional view of some embodiments of a region of the memory device 100 taken alone line A-A′ of FIG. 1A.

The memory device 100 comprises a plurality of first conductive lines 102. The first conductive lines 102 each extend laterally in a first direction. In various embodiments, the first conductive lines 102 are arranged in parallel with one another. In some embodiments, the plurality of first conductive lines 102 may comprise, for example, copper, aluminum, tungsten, some other suitable conductor, or a combination of the foregoing.

A plurality of second conductive lines 104 are arranged over the plurality of first conductive lines 102. The second conductive lines 104 each extend laterally in a second direction transverse the first direction. In various embodiments, the second conductive lines 104 are arranged in parallel with one another. In some embodiments, the plurality of second conductive lines 104 may comprise, for example, copper, aluminum, tungsten, some other suitable conductor, or a combination of the foregoing.

A plurality of one selector-one memory cell (1S1MC) stacks 106 are disposed between the plurality of first conductive lines 102 and the plurality of second conductive lines 104. In various embodiments, the 1S1MC stacks 106 are arranged in an array having a plurality of rows 108 a-c and a plurality of columns 110 a-c. In some embodiments, an individual first conductive line 102 and an individual second conductive line 104 are coupled to each individual 1S1MC stack 106. In further embodiments, respective first conductive lines 102 are coupled to respective columns 110 a-110 c of 1S1MC stacks 106. In yet further embodiments, respective second conductive lines 104 are coupled to respective rows 108 a-108 c of 1S1MC stacks 106.

Each 1S1MC stack 106 comprises a memory cell 112 separated from a threshold selector 114 by an electrode 116. In various embodiments, the electrode 116 is a common electrode directly contacting the memory cell 112 and the threshold selector 114. In some embodiments, the electrode 116 may comprise, for example, doped polysilicon, copper, aluminum, tungsten, some other suitable conductor, or a combination of the foregoing.

The memory cell 112 is configured to store data and may be a non-volatile memory cell or a volatile memory cell. In some embodiments, the memory cell 112 may be a resistive switching memory cell (e.g., resistive random-access memory (RRAM) cell, phase-change random-access memory (PCRAM) cell, conductive-bridging random-access memory (CBRAM) cell, etc.) configured to store data based on a resistive state of a data storage structure. For example, the data storage structure may have a high resistance state associated with a first data state (e.g., binary “0”) or a low resistance state associated with a second data state (e.g., binary “1”). In some embodiments, the data storage structure may comprise, for example, a chalcogenide, an oxide, a nitride, a high-k dielectric, or some other suitable dielectric. In further embodiments, the memory cell may be a magnetoresistive random-access memory (MRAM) cell. In such an embodiment, the data storage structure may comprise a magnetic tunnel junction (MTJ) configured to store data based on a magnetic orientation of the MTJ.

The threshold selector 114 comprises a first amorphous switching structure 120 separated from a second amorphous switching structure 122 by a first buffer structure 124. The threshold selector 114 is configured to switch between a low resistance state and a high resistance state depending on whether a voltage applied across the threshold selector 114 is greater than a threshold voltage. For example, the threshold selector 114 may have a high resistance state if a voltage across the threshold selector 114 is less than the threshold voltage, and the threshold selector 114 may have a low resistance state if a voltage across the threshold selector 114 is greater than the threshold voltage.

In various embodiments, the first buffer structure 124 contacts both the first amorphous switching structure 120 and the second amorphous switching structure 122. In some embodiments, the first buffer structure 124 may comprise an amorphous solid. In further embodiments, the first buffer structure 124 may comprise, for example, amorphous silicon (a-Si), amorphous germanium (a-Ge), amorphous silicon germanium (a-SiGe), an amorphous carbon nitride compound (a-CNx), amorphous carbon (a-C), boron-doped carbon, a silicon oxide compound (SiOx), or an aluminum oxide compound (AlOx). In yet further embodiments, the first buffer structure 124 may not comprise a chalcogenide (e.g., generally a chemical compound comprising a Group 16 element other than oxygen). In some embodiments, the first buffer structure 124 may have a thickness between about 0.5 nanometer (nm) and about 5 nm. In further embodiments, if the first buffer structure 124 comprises an insulator, the first buffer structure 124 may have a thickness less than about 1 nm, such that the first buffer structure 124 does not negatively affect the electrical properties (e.g., current) of the threshold selector 114.

The first amorphous switching structure 120 and the second amorphous switching structure 122 are configured to switch between low resistance states and high resistance states depending on whether a voltage applied across the threshold selector 114 exceeds a threshold voltage. For example, the first amorphous switching structure 120 may have a high resistance state if a voltage across the first amorphous switching structure 120 is less than the threshold voltage, and the first amorphous switching structure 120 may have a low resistance state if a voltage across the first amorphous switching structure 120 is greater than the threshold voltage. In various embodiments, the first amorphous switching structure 120 may contact the first buffer structure 124 and the electrode 116. In some embodiments, the second amorphous switching structure 122 may contact the first buffer structure 124 and an individual first conductive line 102. In further embodiments, the first amorphous switching structure 120 and the second amorphous switching structure 122 comprise a chalcogenide (e.g., generally a chemical compound comprising a Group 16 element other than oxygen). In yet further embodiments, the first amorphous switching structure 120 and the second amorphous switching structure 122 may comprise, for example, tellurium (Te), selenium (Se), sulfur (S), polonium (Po), in combination with, for example, germanium (Ge), silicon (Si), gallium (Ga), indium (In), phosphorus (P), boron (B), carbon (C), nitrogen (N), oxygen (O), etc.

In some embodiments, the first amorphous switching structure 120 and the second amorphous switching structure 122 may have a thickness between about 2 nm and about 10 nm. The ability of the threshold selector 114 to prevent leakage current in its high resistance state and the threshold voltage of the threshold selector 114 are related to an overall thickness of the first amorphous switching structure 120 and the second amorphous switching structure 122 (e.g., a combined thickness of the first amorphous switching structure 120 and the second amorphous switching structure 122). For example, if the overall thickness is less than a minimum thickness (e.g., about 10 nm), leakage current may be high. On the other hand, if the overall thickness exceeds a maximum thickness (e.g., about 30 nm), the threshold voltage may be too high (e.g., outside a range of operating voltages in a typical integrated chip (IC)).

Further, the crystallization temperature of the first amorphous switching structure 120 and the second amorphous switching structure 122 is related to their respective thicknesses. For example, as a thickness of the first amorphous switching structure 120 or the second amorphous switching structure 122 increases, their respective crystallization temperatures decrease. In some embodiments, the crystallization temperature of the first amorphous switching structure 120 and the second amorphous switching structure 122 is great than about 300° C. In further embodiments, the crystallization temperature of the first amorphous switching structure 120 and the second amorphous switching structure 122 is great than about 400° C.

As discussed above, a threshold selector typically has a single amorphous switching structure. For the threshold selector to prevent leakage current in its high resistance state, the amorphous switching structure generally has a thickness that causes the crystallization temperature of the amorphous switching structure to be less than a back-end-of-line (BEOL) fabrication temperature (e.g., about 300° C. or about 400° C.). Because the crystallization temperature is below the BEOL fabrication temperature, the amorphous switching structure may crystallize. If the amorphous switching structure crystallizes during BEOL fabrication, the amorphous switching structure may be formed having a low resistance state and the threshold selector may not prevent leakage current. Thus, the threshold selector may not prevent unintentional activation of unselected cells.

By inserting the first buffer structure 124 between the first amorphous switching structure 120 and the second amorphous switching structure 122, the respective thicknesses of the first amorphous switching structure 120 and the second amorphous switching structure 122 may effectively be reduced. Therefore, the overall thickness of the first amorphous switching structure 120 and the second amorphous switching structure 122 may be increased (e.g., to prevent leakage current) without decreasing the crystallization temperatures of the first amorphous switching structure 120 and the second amorphous switching structure 122. Thus, the first amorphous switching structure 120 and the second amorphous switching structure 122 may have a crystallization temperature greater than the BEOL fabrication temperature. Because the first amorphous switching structure 120 and the second amorphous switching structure 122 may have a crystallization temperature greater than the BEOL fabrication temperature, the first amorphous switching structure 120 or the second amorphous switching structure 122 may not crystallize during BEOL fabrication. Accordingly, the threshold selector 114 may be formed during BEOL fabrication while ensuring the threshold selector 114 have desirable electrical properties (e.g., prevent leakage current and/or have a functional threshold voltage).

FIGS. 2A-2B illustrate various views of some alternative embodiments of the memory device 100 of FIG. 1 in which the threshold selector 114 has additional buffer structures and an additional amorphous switching structure. FIG. 2A illustrates a perspective view of some embodiments of the memory device 100. FIG. 2B illustrates a cross-sectional view of some embodiments of a region of the memory device 100 taken alone line A-A′ of FIG. 2A.

As shown in FIGS. 2A-2B, a second buffer structure 202 is disposed between the first amorphous switching structure 120 and the electrode 116. In various embodiments, the second buffer structure 202 directly contacts both the first amorphous switching structure 120 and the electrode 116. In some embodiments, the second buffer structure 202 comprises the same material as the first buffer structure 124. In other embodiments, the second buffer structure 202 comprises a different material than the first buffer structure 124. In further embodiments, the second buffer structure 202 may comprise an amorphous solid. In yet further embodiments, the second buffer structure 202 is configured to minimize nucleation sites for the first amorphous switching structure 120 due to the electrode 116 being a crystalline metal, which may act as a crystalline promoter for the first amorphous switching structure 120 during subsequent thermal process(es).

A third buffer structure 204 is disposed between the second amorphous switching structure 122 and a third amorphous switching structure 206. In various embodiments, the third buffer structure 204 directly contacts both the second amorphous switching structure 122 and the third amorphous switching structure 206. In some embodiments, the third buffer structure 204 comprises the same material as the first buffer structure 124. In other embodiments, the second buffer structure 202 comprises a different material than the first buffer structure 124. In further embodiments, the third buffer structure 204 may comprise an amorphous solid.

In various embodiments, the third amorphous switching structure 206 is disposed between the third buffer structure 204 and a fourth buffer structure 208. The third amorphous switching structure 206 is configured to switch between a low resistance state and a high resistance state depending on whether a voltage applied across the threshold selector 114 exceeds a threshold voltage. In various embodiments, the third amorphous switching structure 206 may directly contact the third buffer structure 204 and the fourth buffer structure 208. In various embodiments, the third amorphous switching structure 206 may comprise the same material as the first amorphous switching structure 120 and/or the second amorphous switching structure 122. In some embodiments, the third amorphous switching structure 206 may comprise a different material than the first amorphous switching structure 120 and/or the second amorphous switching structure 122. In further embodiments, a crystallization temperature of the third amorphous switching structure 206 is great than about 300° C. In further embodiments, the crystallization temperature of the third amorphous switching structure 206 is great than about 400° C.

In various embodiments, the fourth buffer structure 208 is disposed between the third amorphous switching structure 206 and an individual first conductive line 102. In various embodiments, the fourth buffer structure 208 directly contacts both the third amorphous switching structure 206 and the individual first conductive line 102. In some embodiments, the fourth buffer structure 208 comprises the same material as the second buffer structure 202. In other embodiments, the fourth buffer structure 208 comprises a different material than the second buffer structure 202. In further embodiments, the fourth buffer structure 208 may comprise an amorphous solid. In yet further embodiments, the fourth buffer structure 208 is configured to minimize nucleation sites for the third amorphous switching structure 206 due to the individual first conductive line 102 being a crystalline metal, which may act as a crystalline promoter for the third amorphous switching structure 206 during subsequent thermal process(es).

In various embodiments, the second buffer structure 202, the third buffer structure 204, and the fourth buffer structure 208 may comprise, for example, amorphous silicon (a-Si), amorphous germanium (a-Ge), amorphous silicon germanium (a-SiGe), an amorphous carbon nitride compound (a-CNx), amorphous carbon (a-C), boron-doped carbon, a silicon oxide compound (SiOx), an aluminum oxide compound (AlOx). In yet further embodiments, the second buffer structure 202, the third buffer structure 204, and the fourth buffer structure 208 may not comprise a chalcogenide (e.g., generally a chemical compound comprising a Group 16 element other than oxygen).

In some embodiments, the second buffer structure 202, the third buffer structure 204, and the fourth buffer structure 208 may have respective thicknesses between about 0.5 nanometer (nm) and about 5 nm. In various embodiments, the first buffer structure, 124, the second buffer structure 202, the third buffer structure 204, and the fourth buffer structure 208 may have respective thicknesses that are substantially the same. In further embodiments, the first buffer structure 124 and the third buffer structure 204 may have respective thicknesses that are substantially the same, while the second buffer structure 202 and the fourth buffer structure 208 have respective thicknesses that are substantially the same but different than the respective thicknesses of the first buffer structure 124 and the third buffer structure 204. In yet further embodiments, if the second buffer structure 202, the third buffer structure 204, or the fourth buffer structure 208 comprise an insulator, their respective thicknesses may be less than about 1 nm, such that the electrical properties (e.g., current) of the threshold selector is not adversely affected.

In various embodiments, the third amorphous switching structure 206 comprises a chalcogenide (e.g., generally a chemical compound comprising a Group 16 element other than oxygen). In some embodiments, the third amorphous switching structure may comprise, for example, tellurium (Te), selenium (Se), sulfur (S), polonium (Po), in combination with, for example, germanium (Ge), silicon (Si), gallium (Ga), indium (In), phosphorus (P), boron (B), carbon (C), nitrogen (N), oxygen (O), etc. In further embodiments, the third amorphous switching structure 206 may have a thickness between about 2 nm and about 10 nm. In yet further embodiments, the first amorphous switching structure 120, the second amorphous switching structure 122, and the third amorphous switching structure 206 have respective thicknesses that are substantially the same.

FIG. 3 illustrates a cross-sectional view of some embodiments of an integrated chip (IC) 300 comprising a memory device 100 having a plurality of semiconductor devices each having a plurality of buffer structures and a plurality of amorphous switching structures.

The IC 300 comprises a metal-oxide-semiconductor filed-effect transistor (MOSFET) 302 disposed on a semiconductor substrate 304. The MOSFET 302 comprises a pair of source/drain regions 306 disposed in the semiconductor substrate 304 and laterally spaced. A gate dielectric 308 is disposed over the semiconductor substrate 304 between the individual source/drain regions 306, and a gate electrode 310 is disposed over the gate dielectric 308.

An interlayer dielectric (ILD) layer 312 is disposed over the MOSFET 302 and the semiconductor substrate 304. The ILD layer 312 comprises one or more ILD materials. In some embodiments, the ILD layer 312 may comprise one or more of a low-k dielectric layer (e.g., a dielectric with a dielectric constant less than about 3.9), an ultra-low-k dielectric layer, or an oxide (e.g., silicon oxide). Conductive contacts 314 are arranged within the ILD layer 312. The conductive contacts 314 extend through the ILD layer 312 to the gate electrode 310 and the pair of source/drain regions 306. In various embodiments, the conductive contacts 314 may comprise, for example, copper, tungsten, or some other conductive material.

An interconnect structure 316 is disposed over the ILD layer 312. The interconnect structure 316 comprises a plurality of inter-metal dielectric (IMD) layers 318. A plurality of conductive wires 320 and a plurality of conductive vias 322 are disposed within the IMD layers 318. The conductive wires 320 and conductive vias 322 are configured to provide electrical connections between various devices disposed throughout the IC 300. In some embodiments, the IMD layer 318 may comprise a low-k dielectric layer, an ultra-low-k dielectric layer, or an oxide. In various embodiments, the conductive wires 320 and conductive vias 322 may comprise, for example, copper, aluminum, or some other conductive material

In various embodiments, the memory device 100 is disposed within the interconnect structure 316. In some embodiments, the memory device 100 is disposed within one of the IMD layers 318. In further embodiments, the memory device 100 may be disposed within multiple IMD layers 318. In such an embodiment, the memory device 100 may comprise multiple layers each comprising a plurality of one selector-one memory cell (1S1MC) stacks 106.

In some embodiment, each of the first conductive lines 102 are electrically coupled to an individual MOSFET 302 (e.g., to a source/drain region 306 of each MOSFET 302) via conductive wires 320 and conductive vias 322 that are disposed beneath the memory device 100. In various embodiments, a conductive via 322 and a conductive wire 320 disposed over the memory device 100 are coupled to each of the second conductive lines 104. In further embodiments, the conductive vias 322 and the conductive wires 320 that are coupled to the plurality of second conductive lines 104 may be electrically coupled to other individual MOSFETs 302 (not shown) (e.g., to a source/drain region 306 of the other MOSFET 302).

FIGS. 4-14 illustrate a series of cross-sectional views of some embodiments for forming an integrated chip (IC) comprising a memory device having a plurality of semiconductor devices each having a plurality of buffer structures and a plurality of amorphous switching structures.

As illustrated by FIG. 4, a gate dielectric layer 402 and a conductive layer 404 are formed over a semiconductor substrate 304, such that the gate dielectric layer 402 separates the conductive layer 404 from the semiconductor substrate 304. In some embodiments, the gate dielectric layer 402 may be silicon dioxide, a high-k dielectric, or some other dielectric. In further embodiments, the conductive layer 404 may be doped polysilicon, metal, or some other conductor. In yet further embodiments, the semiconductor substrate 304 may comprise any type of semiconductor body (e.g., monocrystalline silicon/CMOS bulk, silicon-germanium (SiGe), silicon on insulator (SOI), etc.).

In some embodiments, a process for forming the gate dielectric layer 402 and the conductive layer 404 comprises depositing or growing the gate dielectric layer 402 on the semiconductor substrate 304, and subsequently depositing or growing the conductive layer 404 on the gate dielectric layer 402. In further embodiments, the gate dielectric layer 402 may be deposited or grown by thermal oxidation, chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), sputtering, or some other deposition or growth process. In yet further embodiments, the conductive layer 404 may be deposited or grown by CVD, PVD, ALD, sputtering, electrochemical plating, electroless plating, or some other deposition or growth process.

As shown in FIG. 5, the conductive layer 404 and the gate dielectric layer 402 are patterned into a plurality of gate electrodes 310 and a plurality of gate dielectrics 308. In some embodiments, a process for patterning the conductive layer 404 and the gate dielectric layer 402 comprises forming a masking layer (not shown) on the conductive layer 404. In various embodiments, the masking layer may be formed by a spin on process and patterned using photolithography. In further embodiments, the process comprises performing an etch into the conductive layer 404 and the gate dielectric layer 402 with the patterned masking layer in place, and subsequently stripping the patterned masking layer.

As shown in FIG. 6, pairs of source/drain regions 306 are formed within the semiconductor substrate 304 on opposing sides of the gate electrodes 310. In various embodiments, the pairs of source/drain regions 306 comprise a different doping type than adjoining regions of the semiconductor substrate 304. In some embodiments, the pairs of source/drain regions 306 may be formed by an ion implantation process and may utilize a masking layer (not shown) to selectively implant ions into the semiconductor substrate 304. In further embodiments, forming the pairs of source/drain regions 306 may be a self-aligned process that utilizes the gate electrodes 310 as a masking layer.

As shown in FIG. 7, an interlayer dielectric (ILD) layer 312 is formed over the semiconductor substrate 304. In some embodiments, the ILD layer 312 may be formed by CVD, PVD, ALD, or some other deposition or growth process. In further embodiments, a planarization process (e.g., a chemical-mechanical planarization (CMP)) may be performed on the ILD layer 312 to form a substantially planar upper surface.

Also shown in FIG. 7, conductive contacts 314 are formed extending through the ILD layer 312 to the source/drain regions 306 and the gate electrodes 310. In some embodiments, a process for forming the conductive contacts 314 comprises performing an etch into the ILD layer 312 to form contact openings that correspond to the conductive contacts 314. In some embodiments, the etch may be performed with a patterned masking layer formed over the ILD layer 312. In further embodiments, the process comprises filling the contact openings with a conductive material. In yet further embodiments, the contact openings may be filled by depositing or growing a conductive layer covering the ILD layer 312 that fills the contact openings, and subsequently performing a planarization (e.g., CMP) on the ILD layer 312. In various embodiments, the process may be part of a single damascene like process or a dual damascene like process.

As shown in FIG. 8, a lower group of IMD layers 318 are formed over the ILD layer 312. In some embodiments, an individual IMD layer 318 may be formed by CVD, PVD, ALD, or some other deposition or growth process. In further embodiments, a planarization process (e.g., CMP) may be performed on the individual IMD layer 318 to form substantially planar upper surfaces. This process may be repeated multiple time to form each IMD layer 318 of the lower group of IMD layers 318.

Also shown in FIG. 8, a plurality of conductive wires 320 and a plurality of conductive vias 322 are formed within the IMD layers 318. In some embodiments, a process for forming the conductive wires 320 and the conductive vias 322 in an individual IMD layer 318 comprises performing an etch into the individual IMD layer 318 to form via openings and conductive wire openings that respectively correspond to the conductive vias 322 and the conductive wires 320. In some embodiments, the etch may be performed with a patterned masking layer formed over the individual IMD layer 318. In further embodiments, the process comprises filling the openings with a conductive material. In yet further embodiments, the openings may be filled by depositing or growing a conductive layer covering the individual IMD layer 318 that fills the openings, and subsequently performing a planarization (e.g., CMP) on the individual IMD layer 318. This process may be repeated multiple time to form the plurality of conductive wires 320 and plurality of conductive vias 322 in each of the IMD layer 318 of the lower group of IMD layers 318.

As shown in FIG. 9, a first conductive layer 902 is formed over the lower group of IMD layers 318. A first buffer layer 904 is formed over the first conductive layer 902. A first amorphous switching layer 906 is formed over the first buffer layer 904. A second buffer layer 908 is formed over the first amorphous switching layer 906. A second amorphous switching layer 910 is formed over the second buffer layer 908. A third buffer layer 912 is formed over the second amorphous switching layer 910. A third amorphous switching layer 914 is formed over the third buffer layer 912. A fourth buffer layer 916 is formed over the third amorphous switching layer 914. A second conductive layer 918 is formed over the fourth buffer layer 916. A data storage layer 920 is formed over the second conductive layer 918.

In some embodiments, a process for forming the first conductive layer 902, the first buffer layer 904, the first amorphous switching layer 906, the second buffer layer 908, the second amorphous switching layer 910, the third buffer layer 912, the third amorphous switching layer 914, the fourth buffer layer 916, the second conductive layer 918, and the data storage layer 920 comprises depositing or growing the respective layers over one another by, for example, CVD, PVD, ALD, sputtering, electrochemical plating, electroless plating, or some other deposition or growth process.

In various embodiments, the first conductive layer 902 and the second conductive layer 918 may comprise, for example, copper, aluminum, tungsten, some other suitable conductor, or a combination of the foregoing. In some embodiments, the data storage layer 920 may comprise, for example, a chalcogenide, an oxide, a nitride, a high-k dielectric, or some other suitable dielectric. In further embodiments, the first amorphous switching layer 906, the second amorphous switching layer 910, and the third amorphous switching layer 914 may comprise a chalcogenide. More specifically, the first amorphous switching layer 906, the second amorphous switching layer 910, and the third amorphous switching layer 914 may comprise, for example, tellurium (Te), selenium (Se), sulfur (S), polonium (Po), in combination with, for example, germanium (Ge), silicon (Si), gallium (Ga), indium (In), phosphorus (P), boron (B), carbon (C), nitrogen (N), oxygen (O), etc. In yet further embodiments, the first buffer layer 904, the second buffer layer 908, the third buffer layer 912, and the fourth buffer layer 916 may not comprise a chalcogenide. More specifically, the first buffer layer 904, the second buffer layer 908, the third buffer layer 912, and the fourth buffer layer 916 may comprise, for example, amorphous silicon (a-Si), amorphous germanium (a-Ge), amorphous silicon germanium (a-SiGe), an amorphous carbon nitride compound (a-CNx), amorphous carbon (a-C), boron-doped carbon, a silicon oxide compound (SiOx), an aluminum oxide compound (AlOx).

As shown in FIG. 10, a plurality of patterned line structures 1002 a-c are respectively formed over a plurality of first conductive lines 102. While not visible within the cross-sectional view of FIG. 10, the patterned line structures 1002 a-c are elongated/line shaped along axes extending into and out of the page. The first conductive lines 102 respectively separate the patterned line structures 1002 a-1002 c from the lower group of IMD layers 318. In various embodiments, the first conductive lines 102 and the patterned line structures 1002 a-c extend continuously in a first direction. In some embodiments, the first conductive lines 102 and the patterned line structures 1002 a-c respectively extend in substantially straight lines parallel to one another in the first direction.

In some embodiments, a process for forming the patterned line structures 1002 a-c and the first conductive lines 102 comprises forming a masking layer (not shown) on the data storage layer 920. In further embodiments, the process comprises performing an etch into the data storage layer 920, the second conductive layer 918, the fourth buffer layer 916, the third amorphous switching layer 914, the third buffer layer 912, the second amorphous switching layer 910, the second buffer layer 908, the first amorphous switching layer 906, the first buffer layer 904, and the first conductive layer 902 with the patterned masking layer in place, and subsequently stripping the patterned masking layer. In further embodiments, a single etch is performed to form the patterned line structures 1002 a-c and the first conductive lines 102. In other embodiments, multiples etches are performed to form the patterned line structures 1002 a-c and the first conductive lines 102.

As shown in FIG. 11, an intermediate IMD layer 318 is formed over the patterned line structures 1002 a-c and the lower group of IMD layers 318. In some embodiments, the intermediate IMD layer 318 may be formed by CVD, PVD, sputtering, or some other deposition or growth process. In further embodiments, planarization processes (e.g., CMP) may be performed on the intermediate IMD layer 318 to form substantially planar upper surfaces.

As shown in FIG. 12, a third conductive layer 1202 is formed over the patterned line structures 1002 a-c and the intermediate IMD layer 318. In some embodiments, a process for forming the third conductive layer 1202 may comprise depositing the third conductive layer 1202 by, for example, CVD, PVD, ALD, sputtering, electrochemical plating, electroless plating, or some other deposition or growth process. In various embodiments, the third conductive layer may comprise, for example, copper, aluminum, tungsten, some other suitable conductor, or a combination of the foregoing.

As shown in FIG. 13, a plurality of second conductive lines 104 are formed over a plurality of one selector-one memory cell (1S1MC) stacks 106. Note that only one of the second conductive lines 104 is visible within the cross-sectional view of FIG. 13. In various embodiments, the second conductive lines 104 respectively extend in a second direction transverse the first direction. In some embodiments, the 1S1MC stacks 106 are formed as discrete structures that are arranged in a plurality of rows and columns. In further embodiments, the 1S1MC stacks 106 respectively extend between the second conductive lines 104 and the first conductive lines 102 at a plurality of points in which the second conductive lines 104 and the first conductive lines 102 overlap one another in a vertical direction.

In some embodiments, a process for forming the 1S1MC stacks 106 and the second conductive lines 104 comprises forming a masking layer (not shown) on the third conductive layer 1202. In further embodiments, the process comprises performing an etch into the third conductive layer 1202, the patterned line structures 1002 a-c, and the intermediate IMD layer 318 with the patterned masking layer in place, and subsequently stripping the patterned masking layer. In further embodiments, a single etch is performed to form the 1S1MC stacks 106 and second conductive lines 104. In other embodiments, multiple etches are performed to form the 1S1MC stacks 106 and the second conductive lines 104.

As shown in FIG. 14, an upper IMD layer 318 is formed over the 1S1MC stacks 106 and the second conductive lines 104. In some embodiments, the upper IMD layer 318 is part of an upper group of IMD layers 318 comprising a plurality of IMD layers 318. In some embodiments, the upper IMD layer 318 may be formed by CVD, PVD, ALD, or some other deposition or growth process. In further embodiments, a planarization process (e.g., CMP) may be performed on the upper IMD layer 318 to form substantially planar upper surfaces. In further embodiments, formation of the upper IMD layer 318 fills openings between the 1S1MC stacks 106 formed during formation of the second conductive lines 104 and 1S1MC stacks 106, such that the 1S1MC stacks 106 are separated from one another by an IMD layer 318.

Also shown in FIG. 14, a plurality of conductive wires 320 and a plurality of conductive vias 322 are formed within the upper IMD layer 318. In some embodiments, a process for forming the conductive wires 320 and the conductive vias 322 in the upper IMD layer 318 comprises performing an etch into the upper IMD layer 318 to form via openings and conductive wire openings that respectively correspond to the conductive vias 322 and the conductive wires 320. In some embodiments, the etch may be performed with a patterned masking layer formed over the upper IMD layer 318. In further embodiments, the process comprises filling the openings with a conductive material. In yet further embodiments, the openings may be filled by depositing or growing a conductive layer covering the upper IMD layer 318 that fills the openings, and subsequently performing a planarization (e.g., CMP) on the upper IMD layer 318.

The upper IMD layer 318 is formed with a fabrication temperature that is less than the crystallization temperature of the amorphous switching structures (e.g., the crystallization temperature of the first amorphous switching structure 120, the second amorphous switching structure 122, and/or the third amorphous switching structure 206). If the crystallization temperatures of the amorphous switching structures were less than the fabrication temperature, the amorphous switching structures may crystallize, and thus have a low resistance state. Because the buffer structures (e.g., the first buffer structure 124, the second buffer structure 202, the third buffer structure 204, and/or the fourth buffer structure 208) reduce the individual thicknesses of the amorphous switching structures, the crystallization temperatures of the amorphous switching structures may be greater than the fabrication temperature. By utilizing amorphous switching structures and buffer structures in the threshold selector 114, a combined thickness of the amorphous switching structures is sufficient to minimize leakage current through the threshold selector 114 without reducing the crystallization temperatures of the amorphous switching structures below the fabrication temperature.

As illustrated in FIG. 15, a flowchart 1500 of some embodiments of a method for forming an integrated chip (IC) comprising a memory device having a plurality of semiconductor devices each having a plurality of buffer structures and a plurality of amorphous switching structures is provided. While the flowchart 1500 of FIG. 15 is illustrated and described herein as a series of acts or events, it will be appreciated that the illustrated ordering of such acts or events is not to be interpreted in a limiting sense. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. Further, not all illustrated acts may be required to implement one or more aspects or embodiments of the description herein, and one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases.

At 1502, an interlayer dielectric (ILD) layer is formed over a semiconductor device, where the semiconductor device is disposed on a semiconductor substrate. FIGS. 4-7 illustrate cross-sectional views of some embodiments corresponding to act 1502.

At 1504, a first inter-metal dielectric (IMD) layer is formed over the ILD layer. FIG. 8 illustrates a cross-sectional view of some embodiments corresponding to act 1504.

At 1506, a first conductive layer is formed over the first IMD layer, a first amorphous switching layer is formed over the first conductive layer, a buffer layer is formed over the first amorphous switching layer, and a second amorphous switching layer is formed over the buffer layer. FIG. 9 illustrates a cross-sectional view of some embodiments corresponding to act 1506.

At 1508, the second amorphous switching layer, the buffer layer, the first amorphous switching layer, and the first conductive layer are patterned to form a plurality of patterned line structures respectively over a plurality of first conductive lines. FIG. 10 illustrates a cross-sectional view of some embodiments corresponding to act 1508.

At 1510, a second IMD layer is formed over the plurality of patterned line structures and the plurality of first conductive lines. FIG. 11 illustrates a cross-sectional view of some embodiments corresponding to act 1510.

At 1512, a second conductive layer is formed over the second IMD layer. FIG. 12 illustrates a cross-sectional view of some embodiments corresponding to act 1512.

At 1514, the second conductive layer, the second IMD layer, and the patterned line structures are patterned to form a plurality of second conductive lines over a plurality of one selector-one memory cell (1S1MC) stacks, where the plurality of 1S1MC stacks extend between the plurality of first conductive lines and the plurality of second conductive lines. FIG. 13 illustrates a cross-sectional view of some embodiments corresponding to act 1514.

At 1516, a third IMD layer is formed over the plurality of second conductive lines and the plurality of 1S1MC stacks. FIG. 14 illustrates a cross-sectional view of some embodiments corresponding to act 1516.

In some embodiments, the present application provides a semiconductor device. The semiconductor device includes a first amorphous switching layer disposed over a first electrode. A first buffer structure is disposed over the first amorphous switching structure. A second amorphous switching structure is disposed over the first buffer structure. A second electrode is disposed over the second amorphous switching structure, where the first and second amorphous switching structures are configured to switch between low resistance states and high resistance states depending on whether a voltage from the first electrode to the second electrode exceeds a threshold voltage.

In other embodiments, the present application provides an integrated chip (IC). The IC includes a semiconductor device disposed on a semiconductor substrate. An interlayer dielectric (ILD) layer is disposed over the semiconductor substrate and the semiconductor device, where a contact via extends through the ILD layer to the semiconductor device. A threshold selector and a memory cell are stacked over the ILD layer, where the threshold selector includes a first amorphous switching structure separated from a second amorphous switching structure by a buffer structure.

In yet other embodiments, the present application provides a method for forming a semiconductor device. The method includes forming an interlayer dielectric (ILD) layer over a semiconductor substrate. A first conductive layer is formed over the ILD layer. A first amorphous switching layer is formed over the first conductive layer. A buffer layer is formed over the first amorphous switching layer. A second amorphous switching layer is formed over the buffer layer. The first amorphous switching layer, the second amorphous switching layer, and the buffer layer are etched to form a plurality of threshold selectors, where the plurality of threshold selectors are disposed in an array and each threshold selector of the plurality of threshold selectors comprises a first amorphous switching structure separated from a second amorphous switching structure by a buffer structure.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A semiconductor device, comprising: a first amorphous switching structure disposed over a first electrode; a first buffer structure disposed over the first amorphous switching structure; a second amorphous switching structure disposed over the first buffer structure, wherein the first amorphous switching structure has a first thickness, the second amorphous switching structure has a second thickness, and the first buffer structure has a third thickness less than the first thickness and the second thickness; and a second electrode disposed over the second amorphous switching structure, wherein the first and the second amorphous switching structures are configured to switch between low resistance states and high resistance states depending on whether a voltage from the first electrode to the second electrode exceeds a threshold voltage.
 2. The semiconductor device of claim 1, further comprising: a source/drain region disposed in a semiconductor substrate; and an interlayer dielectric (ILD) layer disposed over the semiconductor substrate, wherein a contact extends through the ILD layer to the source/drain region, and wherein the first electrode, the first amorphous switching structure, the first buffer structure, the second amorphous switching structure, and the second electrode are disposed over the ILD layer.
 3. The semiconductor device of claim 1, wherein the first thickness is substantially the same as the second thickness.
 4. The semiconductor device of claim 1, further comprising: a second buffer structure disposed between the first electrode and the first amorphous switching structure.
 5. The semiconductor device of claim 4, wherein the first buffer structure and the second buffer structure have substantially the same thicknesses.
 6. The semiconductor device of claim 4, further comprising: a third buffer structure disposed between the second amorphous switching structure and a third amorphous switching structure; and a fourth buffer structure disposed between the third amorphous switching structure and the second electrode.
 7. The semiconductor device of claim 6, wherein the third and the fourth buffer structures have substantially the same thicknesses as the first buffer structure and the second buffer structure, and wherein the third amorphous switching structure has substantially the same thickness as the first and the second amorphous switching structures.
 8. The semiconductor device of claim 6, wherein the fourth buffer structure contacts the second electrode and the third amorphous switching structure, the third buffer structure contacts the second amorphous switching structure and the third amorphous switching structure, the first buffer structure contacts the second amorphous switching structure and the first amorphous switching structure, and the second buffer structure contacts the first electrode and the first amorphous switching structure.
 9. The semiconductor device of claim 1, wherein the first amorphous switching structure and the second amorphous switching structure have a thickness between about 2 nanometers (nm) and about 10 nm.
 10. The semiconductor device of claim 9, wherein the first buffer structure has a thickness between about 0.5 nm and about 5 nm.
 11. An integrated chip, comprising: a semiconductor device disposed on a semiconductor substrate; an interlayer dielectric (ILD) layer disposed over the semiconductor substrate and the semiconductor device, wherein a contact via extends through the ILD layer to the semiconductor device; and a threshold selector and a memory cell stacked over the ILD layer, wherein the threshold selector comprises: a first amorphous switching structure disposed over an electrode; a first buffer structure disposed over the first amorphous switching structure; a second amorphous switching structure disposed over the first buffer structure; and a second buffer structure disposed between the first amorphous switching structure and the electrode.
 12. The integrated chip of claim 11, wherein the first amorphous switching structure is configured to switch between a low conductivity state and a high conductivity state depending on whether a voltage across the threshold selector exceeds a threshold voltage.
 13. The integrated chip of claim 11, wherein the first amorphous switching structure and the second amorphous switching structure comprise a chalcogenide.
 14. The integrated chip of claim 11, wherein the first buffer structure does not comprise a chalcogenide.
 15. The integrated chip of claim 11, wherein the first buffer structure has a crystallization temperature greater than about 300° C.
 16. The integrated chip of claim 11, wherein the first buffer structure is an amorphous solid.
 17. The integrated chip of claim 11, further comprising: a plurality of first conductive lines disposed over the ILD layer, wherein the plurality of first conductive lines extend in a first direction; a plurality of second conductive lines disposed over the plurality of first conductive lines, wherein the plurality of second conductive lines extends over the plurality of first conductive lines in a second direction transverse the first direction; and wherein the memory cell is one of a plurality of memory cells and the threshold selector is one of a plurality of threshold selectors disposed in an array between the plurality of first conductive lines and the plurality of second conductive lines, and wherein the plurality of first conductive lines are coupled to the plurality of threshold selectors and the plurality of second conductive lines are coupled to the plurality of memory cells.
 18. The integrated chip of claim 11, wherein the second buffer structure contacts both the electrode and the first amorphous switching structure.
 19. A method for forming a semiconductor device, the method comprising: forming an interlayer dielectric (ILD) layer over a semiconductor substrate; forming a first conductive layer over the ILD layer; forming a first amorphous switching layer over the first conductive layer; forming a buffer layer over the first amorphous switching layer; forming a second amorphous switching layer over the buffer layer; etching the first amorphous switching layer, the second amorphous switching layer, and the buffer layer to form a plurality of threshold selectors, wherein the plurality of threshold selectors are disposed in an array and each threshold selector of the plurality of threshold selectors comprises a first amorphous switching structure separated from a second amorphous switching structure by a buffer structure; and forming an interconnect structure over the ILD layer, wherein the plurality of threshold selectors are disposed within the interconnect structure, wherein formation of the interconnect structure comprises subjecting the semiconductor substrate to a fabrication temperature greater than about 400° C., and wherein the first amorphous switching structure and the second amorphous switching structure remain amorphous solids above the fabrication temperature.
 20. The method of claim 19, wherein: the first amorphous switching layer has a first thickness; the second amorphous switching layer has a second thickness; and the buffer layer has a third thickness less than the first thickness and the second thickness. 